Fanconi anemia (FA) is a complex genetic disorder characterized by a progressive bone marrow aplasia, chromosomal instability, and the acquisition of malignancies, particularly myeloid leukemia. The cDNAs of eight FA genes have been sequencea, including FANCA and FANCC, which collectively account for 75% of all FA patients. The identification of these genes raises the potential of using gene transfer technology io express the functional cDNA into autologous stem cells. The use of murine models containing a disruption of the murine homologue of FA genes presents a major opportunity to examine in detail the pathogenesis of this hematopoietic disease, to study hematopoietic function following insertion of the correct cDNA into the cell in an in vivo system, and to evaluate novel transplantation protocols. In the previous funding period, our group developed two highly reproducible models that allowed us to test the potential of gene therapy to restore stem cell repopulating ability as well as protect stem cells from the acquisition of myeloid leukemias. Importantly, we demonstrated that introduction of recombinant Fancc into Fancc -/- stem cells enhances repopulating ability to near wildtype levels and protects the reconstituted mice from the acquisitionof hematopoietic malignancies found in 30-40% of Fancc -/- mice reconstituted with cells containingreporter gene sequences only. Furthermore, we demonstrated that Fancc -/- stem/progenitor cells are hypersensitiveto apoplosis during a 4 day ex vivo culture, and in the absence of gene correction, mice reconstitutedwith the residual Fancc-/- stem cells have a high incidence of myeloid malignancies and cytogenetic abnormalities. Based on these observations, as well as observations that previous attempts to use retroviral vectors in FA patients resulted in no clinically significant or long term engraftment, we propose to utilize these recently developed models to test the potential 9f foamy viruses and lentiviruses to express recombinant Fancc in vivo. A key advantage of these two gene delivery systems is that they do not require prolonged ex vivo culture. We propose to determine whether these gene delivery systems can restore stem cell function and also prevent the development of myeloid leukemia in Fancc -I- mice. We also propose to extend these studies in Fanca deficient mice, since mutations of FANCA account for 50-60% of all instances of Fariconi Anemia patients. Finally, it has become apparent that some method of myelopreparation or in vivo selection is required for autologous engraftment of genetically corrected cells in FA patients and FA mutant mice. We and others have previously demonstrated that Fancc deficient bone marrow cells are hypersensitive to inhibitory cytokmes, including IFN-y/n vitro in Fancc -/- mice. In this application, we propose to extend those observations in vivo in Fancc -/- mice as well as Fanca -/- mice and evaluate whether IFN-y can enhance the engraftment of exogenous genetically corrected stem cells.